High resolution synthetic aperture radar (SAR) is used for detail ground mapping at long range. The data array for the SAR image, which has been motion-compensated to produce the focused image, may still contain residual phase errors which result in an unfocused or smeared image. A number of effects may create this error such as turbulence, errors in the velocity or inertial platform data from the aircraft and variations in the height of ground features. However, the dominant source of error is an erroneously sensed motion of the aircraft, caused by the inherent limitation of the inertial navigation system.
An automatic estimation and compensation of the phase error has classically been obtained using several techniques. The residual phase error is assumed to be representable in terms of the second or higher order polynomial and each autofocus technique attempts to estimate the coefficients of the assumed polynomial function. This phase error estimation usually involves partitioning the SAR array into several subarrays. One example is the phase comparison method found in U.S. Pat. No. 4,219,811 to Herman et al. The Herman technique creates a vector resultant from each of three subarrays formed from the SAR array. A phase correction term is derived by comparing the phase angle of the first end subarray vector resultant with respect to the other end subarray vector resultant, bisecting the angle of the two resultants and comparing it with the phase of the central subarray resultant. Another example is the map drift autofocus method as discussed by C. E. Mancill and J. M. Swiger, published in the 27th Tri-Service Radar Symposium Records June 1981. In this method, the multiple lower resolution images are produced from subarrays and are correlated to determine the relative shift. A set of relative shifts or drifts among subarrays are then processed to yield the coefficient of the assumed polynomial.
The phase comparison method is usually used for a quadratic phase correction and has a pull-in range of approximately 180 degrees. The map drift method has a very large pull-in range and has been known to estimate the higher order phase errors reliably.
Focus corrections provided by either method described will significantly improve the SAR image, provided that the underlying phase error can be accurately represented by a polynomial. However, those methods start to break down if the actual phase error is highly non-linear and requires a very high order polynomial for its accurate representation.
One technique that is different from the techniques described above is a method where one attempts to extract the reference phase signal directly from a target in the image. However, all known techniques of this type involve the extraction of the phase data, unwrapping of the phase data to create a continuous phase function, followed by some type of a polynomial fitting.
The present invention method provides a means for extracting the phase reference data from an isolated point target using this novel approach, and at the same time eliminates the need for phase unwrapping and higher order polynomial curve fitting.